[fjtorres]

The pocketbook firmware lets you customize the display and render the text with any font at any size. It also has an embolden option.
The 301+ is a good option, as pointed out above.
The 360 might be too small (It is the exact size of a CD jewel case), but it has an optional reader app that lets you switch pages by moving the reader itself instead of pressing a button. It might suit her needs in landscape mode.
The 302 has a touch screen and a motion sensor with a 6 in screen, plus large paging buttons on both sides. It might give you the best of the two other models but it is a bit on the pricey side by current standards and, for the same price, they are going to be offering a fairly light 9in model starting around september, the PB901.
There are threads on all four models over at the Pocketbook forum.

On the low-end of the scale, the Kobo might be an option at (US$150); the hardware is very similar to the 301 but it uses different, simpler/leaner firmware.

The Kindle's new firmware added two more larger font sizes so it might also work for you.
My mother is doing okay with the older Kindle firmware at the largest size. Biggest issue with the K2 is the tiny size of the 5-way controller button.

You may also want to check out the user interface on the Sony PRS-600; it has the touchscreen at US$199 these days.